Stains are unsightly discolored or soiled spots or smudges that occur practically anywhere. Common sources of stains are oil, grease, paint, rust, mold, plants, food and beverages, bird droppings, tire marks, and the like. In most cases, a stain is a result of a chemical reaction or strong interaction, such as, adsorption or chemisorption, between a staining agent and the surface of a solid material.
Lubricating oil (lube-oil) stains on concrete and asphalt driveways are among the most common stains and, unfortunately, most resistant stains. This is due to their high molecular weight and viscosity, low solubility in common solvents and pronounced hydrophobic properties. Chemically, lube-oils present a very complex mixture of long-chain hydrocarbons (>C20) and a number of additives including oxidation inhibitors (to prolong the life of the lube-oil), anti corrosion agents (to protect metal from sulphides and other corrosive elements), guards against scoring and galling, anti wear agents (to prevent abrasive and metal to metal contact) and others.
Used lube-oil also contains heavy metals, such as, nickel (Ni), copper (Cu), chromium (Cr), vanadium (V), oxygenates, sulphur compounds and other impurities. Due to an intrinsic chemical inertness of lube-oils, the requisite property of the lube-oil, and the presence of an oxidation inhibitor a lube-oil stain may remain intact for an extended period of time, months and even years. All these factors add to the difficulty of treating and removing lube-oil stains, especially, from rough surfaces.
A rust stain is another common and unforgiving adversary for concrete and other rugged surfaces. Rust stains mainly result from two sources. A first source of rust is water running over metal such as iron and steel and depositing metal oxide particles on the paving or other surfaces. A second source is oxidation of iron-rich compounds in the aggregate of the paving or other surfaces. In principle, rust can be removed via reactions with phosphoric, muriatic, oxalic acids and other acids. Most commercial rust removers contain acids. However, any chemical agent, especially, acid-based, that treats the rust stain would potentially affect the material of the surface sometimes leading to its discoloration or even damage. The stains are particularly difficult to remove from rugged, rough porous surfaces such as those typically occurring in concrete, brick, limestone and other materials. Cement is a versatile and widely used paving and building material, but it is porous and has a very high surface area as reported by V. Ramachandran, et al. in “Removal of Stains from Concrete Surfaces,” Canadian Building Digest. Report CBD-153, (1973). Stains tend to soak into cement on contact and in some cases may react with the hydrated cement. Some stains, particularly old ones, so strongly adhere to the porous surface of cement that they will resist to any efforts to remove them. As a result, removal of old oil stains from rugged surfaces like those occurring on cement, brick and other porous materials becomes extremely difficult and requires a special treatment.
Known procedures for removing stains from rugged solid surfaces, such as, concrete, include mechanical and chemical methods or a combination of both methods. Mechanical methods include sand blasting, grinding, steam cleaning, brushing, scouring and use of blow torches. In most cases, these procedures are cumbersome, labor intensive and rather expensive, and do not always give a desirable cleaning effect. Depending on the age and severity of the stain these methods may require multiple treatments to get results, which could potentially compromise the integrity of the surface subjected to cleaning.
Existing chemical methods of stain removal from solid surfaces involve the use of special chemicals, solvents, detergents, enzymes and other biological agents. Solvents and detergents dissolve stains, whereas chemical and biological agents react with them leading to their degradation or formation of a compound that will not show as a stain.
The prior art describes a variety of chemical methods for removing organic stains from solid surfaces. For example, Matsui et al. in Japanese Patent JP 10176427 teaches a method for removing oil stain from stones. The method includes coating the oil stain with a paste containing a lipophilic organic solvent and solid powders, covering the surface of the stone by a film, which does not permeate vapors of the solvent, adhering the covering with tapes to prevent the vapor from scattering.
Japanese Patent JP 08188893 to Asakawa describes aqueous detergents for removing oil stains from metal objects. The method involves the use of aqueous detergents containing fat-decomposing enzymes and nonionic surfactants at a ratio of (1-100):(1-1000) and have pH 6.5-10.
European Patent EP 441481 to Kerze discloses a method for cleaning hydrocarbon-stained hard surfaces. The method includes surrounding the stain on the surface with means for containing a liquid in contact with the stain, placing in the containment an organic solvent that is miscible with petroleum hydrocarbons, maintaining the solvent in contact with stain for 1-24 hours, removing the solvent, placing on the surface an adsorbent that does not dissolve the solvent, maintaining the adsorbent in contact with the surface for an additional 1-24 hours, removing the adsorbent material and allowing the treated surface to dry. The author claims that the method is effective in removing motor oil stains from concrete surfaces.
An absorbent composition for removing oils and greases from driveways is described by Hatton in European Patent EP 260135. The absorbent composition comprises an organic material derived from plant gums and plant mucilages and it is applied as a powder, paste or sheet to the liquid spill or the layer to be absorbed. Thus, ground Plantago psyllium husk powder was applied to a patch of oil on a concrete surface resulting in removal of the oil after several hours.
Canadian Patent CA 978839 to Parent describes the liquid compositions for removing rust stains from concrete, bricks and ceramics. The composition consists of concentrated phosphoric acid (H3PO4) (98 wt. %) diluted after mixing with ≦0.5% of 1,3-dibutylthiourea and/or 1,3-diethylthiourea and ≦0.1% of a nonionic surfactant.
Uemura in Japanese Patent JP 52069827 teaches a method for rust removal using a paste-like agent. The agent comprises silicon dioxide (SiO2) (50-95%), water and/or alcohol, surfactant and a H3PO4 compound. The rust-removing agent is applied to a metal surface, solidified after 5-10 hours, and easily removed the rust by brushing.
An overview of different methods for removing oil and rust stains from mortar and concrete specimens was reported by Derrington et al. “Removing stains from mortar and concrete.” U.S. Govt. Res. Develop. Rep 1969, 69/5, 88). It was reported that the most effective method for the removal of rust stains is sandblasting and an application of an oxalic acid solution or sodium citrate-sodium dithionite paste. An application of benzene-calcium carbonate (C6H6—CaCO3) paste was reportedly the most effective removal method for oil stains among the techniques that were tried. The report also stated that neither of the known methods could completely remove oil and asphalt stains from concrete surfaces.
Thus, the existing chemical approaches to removal of resistant stains from rugged solid surfaces offer rather labor-intensive and costly techniques that involve the use of expensive organic solvents, biological agents (enzymes), special detergents, surfactants, or corrosive agents, such as acids. Moreover, some solvents, such as, benzene, and acid-based agents may have an undesirable environmental footprint. Furthermore, after the stain treatment by the agents, including solvents, detergents, acid-based formulations, and the like, they have to be washed away by copious amounts of water to prevent a secondary staining.
It should also be emphasized that most of the forementioned chemical methods, especially, those involving the use of poultices or acid-based formulations, are designed for the treatment of horizontal surfaces and may not be applicable for vertical surfaces. In summary, these methods are cumbersome and costly and in many cases do not give good results, especially in treatment of old lubricant-oil and asphalt stains, making them temperamental or even impractical for some applications.
Turning now to photocatalytic methods for the removal of organic contaminants and oil spills from a variety of solid and liquid (e.g., seawater) surfaces, the prior art provides the following examples.
U.S. Patent Publ. No. 2004/0149307 to Hartig describes a self-cleaning reversible window assembly with photocatalytic (e.g., TiO2) layers deposited on opposite sides of a transparent substrate (glass) so that when exposed to sunlight, the photocatalytic layers chemically degrade organic contaminants deposited on the transparent surface. Similarly, U.S. Pat. No. 6,680,135 to Boire et al. relates to transparent surfaces (e.g., glass), which are furnished with photocatalytic coatings that impart “dirt repellent” properties to the surface.
The transparent TiO2 photocatalytic coatings according to Hartig's and Boire's inventions are produced from titanium-based precursor compounds and would require special equipment and sophisticated application techniques such as magnetron sputtering, pyrolytic coating, chemical vapor deposition, cathodic sputtering, sol-gel techniques, etc. These methods, however, may not be suitable for the application of photocatalytic coatings on out-door surfaces, such as concrete driveways, asphalt pavement, etc.
U.S. Patent Publ. No. 2004/0255973 to Chen discloses a method for cleaning the surface of a semiconductor wafer (after chemical/mechanical polishing) comprising the steps of applying a photocatalyst (e.g., TiO2) containing suspension to the surface and exposing it to UV light (wavelengths less than 380 nm).
U.S. Pat. No. 6,645,307 to Fox et al. discloses cleaning compositions comprising a photocatalytic material (colloidal suspensions of TiO2), a sensitizer (e.g., cationic dye/borate anion complex) and a number of other ingredients. The composition combats soils and/or undesired malodours and microorganisms on fabrics and hard surfaces. The TiO2 colloidal suspensions are produced from precursor materials, e.g., by hydrolysis of tetrachloride or titanium isopropoxide. Similarly, U.S. Patent Publication No. 2004/0023824 to Zeuchner et al. applies insoluble solids such as silica, zinc oxide or TiO2 in solution to hard surfaces such as stone, ceramics, wood and the like to increase hydrophilic properties of the surface so that the removal of soil and soil-release are improved.
U.S. Pat. No. 6,013,372 to Hayakawa, et al. describes a surface coated with an abrasion-resistant photocatalytic coating comprised of a semiconductor photocatalyst, such as TiO2. The coating, when exposed to sunlight, makes the surface hydrophilic and self-cleaning when subjected to rainfall.
U.S. Pat. No. 6,659,520 to Jacobs provides a coating composition for preventing algal growth on building materials. The coating composition is an aqueous slurry containing a silicate binder (e.g., sodium silicate) and a plurality of photocatalytic (e.g., TiO2) particles (particle size 1-1000 nm). The composition is heated, up to 650° C., to produce a ceramic-type coating on granular substrates such as rock, clay, ceramic, concrete, and the like.
U.S. Pat. No. 6,409,821 to Cassar et al. describes a hydraulic binder and cement compositions containing photocatalyst (e.g., TiO2) particles having the improved property of maintaining brilliance and color for architectural concretes. The amount of photocatalyst particles in the composition varied in the range of 0.01-10% by weight with respect to the hydraulic binder.
U.S. Pat. No. 6,337,129 to Watanabe et al. discloses a surface layer composition comprising a photocatalyst (e.g., TiO2), a hydrophobic portion (a water repellent fluororesin) and a hydrophilic portion (a silicone resin or silica) in a solvent (water, ethanol, or propanol). The mixture is spray-coated on the substrate surface and the coating is heat treated at 200-380° C. to form a durable surface layer.
U.S. Pat. No. 5,643,436 to Ogawa et al. describes a metal oxide (e.g., TiO2) layer exhibiting a photocatalytic and anti-soiling function on the surface of inorganic architectural materials (e.g., glass, tile, concrete, stone, etc.). Transition metals (Pt, Pd, Ni, Ru, etc.) or their oxides can be added to promote the photocatalytic reaction. Heat treatment (100-800° C.) is necessary to bind the photocatalyst film to the architectural material.
U.S. Pat. No. 5,547,823 to Murasawa et al. discloses a photocatalyst composite comprising a substrate with the photocatalyst (TiO2) particles and the second component (V, Fe, Co, Ni, et.) adhered to a solid surface (glass, ceramics, wood, metals, etc.) using less degradative adhesive (a flu-orinated polymer, a silicon-based polymer, cement, etc.). TiO2 particles are produced from precursors such as titanyl sulfate, titanium tetrachloride, titanium alkoxides and the like by a number of techniques: thermal hydrolysis, neutralization, vapor-phase oxidation. The fixation of the composition to the substrate is accomplished by UV-irradiation, thermal treatment (at temperatures up to 400° C.), using a cross linking agent, etc.
Schutt in WO 2003022462 describes titanium alkoxide-polysiloxane-based coatings with photocatalytic self-cleaning properties. A substrate is coated with a polysiloxane-based coating having titanium oxy-groups from a titanium alkoxide bonded to the polysiloxane backbone. The coating gives the substrate self-cleaning properties upon exposure to UV radiation causing organic stains to spontaneously disappear. Upon exposure to UV radiation, photocatalytically induced oxidation reaction causes the spontaneous disappearance of food, oil and other organic stains on buildings and paving materials, such as concrete, limestone or similar surfaces.
Japanese Patent JP 2000015110 to Nakamura et al. describes the method for the removal of stains on kitchenware. The method utilizes the photocatalyst particles comprising TiO2 grains with an average diameter of equal or less than 1 μm which are partially coated (1-20%) by a metal or its alloy selected from Ru, Pt, Au, Cr.
U.S. Pat. No. 5,604,339 to Raissi et al discloses a method for photocatalytic destruction of harmful volatile compounds at emitting surfaces. In a preferred embodiment, a piece of formaldehyde laden wood substrate, for example, paneling or furniture, is treated with TiO2 suspension to form a thin and translucent veneer, which acts as a membrane preventing outward transport of formaldehyde and other harmful compounds produced by weatherization and natural degradation of the substrate.
In addition to the above, another series of patents deal with the photocatalytic degradation of oil spills on seawater. For example, U.S. Pat. No. 4,997,576 to Heller et al. discloses the method for photocatalytic treatment of an oil film (or spill) floating on seawater, which includes dispersing TiO2-coated water-floatable beads (an average diameter of ≦2 mm) on the oil film and allowing the beads to be exposed to solar illumination and air so that the beads accelerate the oxidation of organic compounds in the oil.
Japanese Patent JP 11300349 to Tokita describes a method for cleaning water surface contaminated by oil spill. The method is carried out by floating on the contaminated water surface a water-soluble substrate coated with thin-film semiconductor photocatalyst and porous material (e.g., zeolite), to be contacted with oil pollution for decomposition under light (solar or lamp) radiation.
Japanese Patent JP 10310779 to Takutani describes photocatalyst devices for treatment and recovery of oil spill on seawater. The devices are made of honeycomb-type photocatalysts having multiple cell holes and TiO2 coatings, and the devices are equipped with floats. Oil spills are photocatalytically oxidized and decomposed under solar light radiation.
In summary, the prior art methods offer a variety of approaches, the majority utilize TiO2 as a photocatalyst, to degrade and remove organic contaminants and stains from different surfaces. Potential problems associated with these approaches are as follows. First, most of the techniques utilize rather costly starting materials, such as, metal-organic precursors, polymeric adhesives, micro-beads, and the like. Second, the majority of methods used require special equipment and elaborate preparation techniques to produce the photocatalytic coatings, including, but not limited to, metal-organic synthesis, chemical vapor deposition, magnetron sputtering. Third, in many cases, the binding of the photocatalyst to the treated surfaces requires thermal treatment (100-600° C.), which is not technically feasible for out-door applications in locations, such as, driveways, pavements, exterior walls.
Moreover, the prior art techniques are not conducive to the degradation of organic contaminants occurring on rugged porous surfaces, where an intimate contact between a photocatalytic agent and the stain surface, including the portion hidden in the micro-pores and micro-crevices, is required. Furthermore, there is no indication that the prior art methods could provide an immediate (i.e., right after the initial treatment) masking effect on the stains making them less visible against the surroundings. Lastly, the above photocatalytic techniques are designed for the treatment of different organic stains and are unlikely to treat and remove inorganic stains, such as those caused by rust.
It is also known that conventional paints contain titanium dioxide and other pigments. However, conventional paints are not useful for masking and removing stains for several reasons. First, paints can provide only masking effect, but they do not remove the stains. Second, paints contain a vehicle (such as oil- or lacquer- or latex-based vehicles), in which the pigment is suspended. The vehicle dries and hardens to form an adhesive film of paint on the surface. This film is practically permanent and over extended period of time (months and years) its masking effect might diminish as the surroundings change color due to aging and exposure to traffic. Thus, paints are not useful for masking and removing stains.
Thus, there is a need for an efficient, simple, user-friendly, care-free and environmentally benign method for the removal of a variety of tough stains from rugged porous surfaces (both horizontal and vertical) such as concrete, stone, brick, stucco, and the like. It is also highly desirable that the method provides an immediate masking effect on the stain via blending the stain with the surrounding area and making it practically invisible.